Bringing Sibling on a Standard Visitor Visa

I'm applying for my parents on a Standard Visitor Visa to attend my graduation ceremony. I have provided them with an invitation letter, while my uncle has given them another letter confirming that he will provide accommodation and financial support.

I also wish to include my sibling, who is a medical student and studying in our home country. However, I am concerned that this may affect my parents' visa application. As we do not have any other family members back home, I would appreciate any advice on how to make the application process successful.

My understanding is that they process every application separately even though the family members are traveling together.